Quote:
Originally Posted by DaveTheReefGeek
What is the calcium carbonate level in your tank?
If your at your max, then no matter how much Ca you add, it may be getting converted.
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Calcium carbonate level? Just curious, how do you determine total solubility of calcium carbonate without considering in seawater what the concentration of the individual dissociate ion concentrations are? What about salinity? pH? Mg++? pCO2 and dissolved CO2concentrations are???
Total alkalinity (
AT) is so closely associated with pCO2 (the partial pressure of CO2 gas), pH, and the sum of the concentrations of the dissolved CO2 in seawater (
ZCO2 -also called the total inorganic carbon dioxide dissolved in seawater) that knowing any two values of the above and the normal STP considerations for surface seawater, we can calculate the other two values. See/Google the Revelle factor* for the relationship between pCO2 and
ZCO2, a ratio affected by the rate of CO2 exchange at the air-seawater interface. We see the effect of the Revelle factor in our tanks when we have chronically low pH readings for our tanks that end up depleting our alkalinity reserves. We can use the exchange rates affecting the Revelle factor to our advantage when alk gets too high in addressing the equilibrium between the inorganic carbon species in the carbonate-
bicarbonate buffer system (alkalinity). By gassing off CO2 via agitation and dropping the Revelle factor, we can remove some of the dissolved CO2 and pCO2 from the tank and equilibrium then shifts from the carbonate end of the equilibrium toward the COOH <---> CO2 end to reduce alk of the tank water, but if Calcium is low and alk is high, we can instead exceed the supersaturation quotient (
W) for calcium with respect to carbonate and add calcium ions to the seawater so that calcium carbonate will precipitate out of solution abiotically, driving the equilibrium to the opposite end of the equilibrium reaction. This will reduce both ion concentrations in an equimolar manner to drop the A
T by an amount equal to the molar quantity of calcium we add.
This most likely occurs as calcite at aquarium temps and pressures, but the total solubility of calcium carbonate (K
sp) will depend more on the ratio of calcite to aragonite in the water column, and this does not even address the issues associated with ion concentrations of calcium and the several carbonate species of ions. Equilibrium depends on calcium carbonate solubility and rate of ionization and solubility as an end product only in the carbonate-
bicarbonate buffer system.
Rather than type it out tonight, see:
http://<br />
http://scifun.chem.wi...k/CO2/CO2.html for the equilibrium dynamics of CO2 in aqueous solutions, and see the following links for more specifics re: alk and calcium in seawater:
http://www.thereeftank.com/forums/sh...&threadid=9024
http://www.thereeftank.com/forums/sh...d.php?t=105458
http://www.thereeftank.com/forums/sh...ad.php?t=13878
http://www.thereeftank.com/forums/sh...ad.php?t=73397
Personally, I think no more additives and several weeks of 20% water changes will bring all the aspects of your water column back to NSW parameters, and this will fix your tank issues. Get some good test kits and learn how to use them, then watch for trends rather than actual numbers. If trends show you dropping alk values, then test calcium and see where you are for a few weeks. Do the same if your alk shows a gradual increase over time. Look at your corals and see how they look, if there are no changes, then don't worry so much. Always treat the patient, not the numbers.
HTH
PS, it is snowing like crazy here tonight (Wednesday nite), the satellite is out, so this will not be posted until tomorrow some time.
*
this is usually controlled by both the temperture of the water and the activity of both wind and water agitation at the surface. The Revelle factor (R) is the relationship (ratio) between the fractional changes (increases usually) in partial presure of CO2 in the atmosphereto the corresponding changes in the mounts of total dissolved CO2 in the seawater:
R = (DPCO2/PCO2)/(DZCO2/ZCO2)
The increase in total CO2 near the equator are related to the Revelle factor. The value is about 14 for cold polar waters and around 8 or so in warmer circumtropical waters, with an oceanic average af around 10. As you can easily see, this would mean that a 10% change in pCO2 ( the value for DPCO2/PCO2) would only result in a 1% change in ZCO2. This factor then becomes important when calculating the effect that atmospheric increases in CO2 have on the carbonate-bicarbonate buffer system in the ocean and overal oceanic acidity.
M
